The precise measurement and control of viscosity in fertilizers and agrochemicals are fundamental for ensuring formulation stability, accurate application rates, and optimal crop uptake. Because the viscosity of liquid fertilizers and pesticide suspensions directly influences droplet size during atomization, laboratory professionals must characterize flow behavior to prevent drift and ensure equipment compatibility. Variations in rheological properties can lead to nozzle clogging, uneven field distribution, or rapid sedimentation of active ingredients in storage. By employing standardized testing methods and understanding non-Newtonian fluid dynamics, formulators can predict how a product will behave under the high-shear conditions of a spray boom versus the low-shear environment of a storage tank.

Analyzing viscosity profiles in suspension concentrates and emulsions

Suspension concentrates (SCs) and emulsions typically exhibit non-Newtonian flow behavior, meaning their viscosity changes depending on the applied shear rate. Laboratory analysis must distinguish between Newtonian fluids, such as simple urea solutions, and complex pseudoplastic formulations that thin under stress. Accurate rheological profiling ensures that a product remains viscous enough at rest to prevent particle settling but becomes fluid enough to pump and spray when agitated.

Shear-Thinning and Thixotropy: Most agrochemical suspensions are designed to be shear-thinning, or pseudoplastic. In this state, the formulation’s viscosity decreases significantly as it is pumped or sprayed. This behavior facilitates efficient movement through application equipment while allowing the fluid to recover structure once it hits the target leaf surface.

Thixotropy is a related but time-dependent property often engineered into high-quality fertilizers. A thixotropic fluid will thin under constant shear and then take a measurable amount of time to rebuild its viscosity after the shear is removed. This recovery time helps prevent run-off from plant foliage immediately after application.

Yield Stress and Sedimentation Control: Yield stress is the minimum amount of force required to initiate flow in a fluid. In the context of suspension fertilizers, a formulated yield stress acts as a net to hold solid particles in suspension against the force of gravity. If the yield stress is too low, the active ingredients will settle into a hard cake at the bottom of the container.

Formulators use rheological modifiers, such as xanthan gum or clay-based thickeners, to tune this yield stress. The goal is to balance physical stability with pourability. Instruments like rotational rheometers are essential for quantifying yield stress values (typically measured in Pascals) to predict shelf-life stability.

Newtonian Baselines in Liquid Fertilizers: Clear liquid fertilizers, such as ammonium polyphosphate or urea-ammonium nitrate (UAN), often display Newtonian behavior. For these fluids, viscosity remains constant regardless of the shear rate applied during handling. However, even Newtonian fertilizers can experience viscosity spikes at lower temperatures, affecting pump sizing and flow meter accuracy.

Establishing a Newtonian baseline allows for simpler quality control testing using kinematic viscosity methods, consistent with the FAO Manual on Development and Use of FAO and WHO Specifications for Pesticides. Yet, when additives or micronutrients are introduced—often adhering to CPDA Adjuvant Certification Program Guidelines—the fluid may transition to non-Newtonian behavior. Continuous monitoring of this transition is vital, particularly when characterizing nanoscale interactions as defined in ASTM E2456.

How viscosity affects spray application and droplet dynamics

Viscosity is the primary determinant of how a liquid stream breaks up into droplets as it exits a spray nozzle, a process known as atomization. Higher viscosity fluids resist the formation of the "ligaments" that eventually snap into individual droplets. Consequently, increasing the viscosity of a spray solution generally shifts the Volume Median Diameter (VMD) of the spray cloud toward larger, coarser droplets.

Drift Reduction and Droplet Size: Controlling spray drift is a major regulatory and environmental priority in modern agriculture. Finer droplets (typically under 150 microns) are prone to moving off-target due to wind, causing damage to neighboring crops. Formulations with higher extensional viscosity are less likely to produce these drift-prone fines.

Drift Reduction Adjuvants (DRAs) are often added to tank mixes to modify the viscosity profile specifically during the atomization phase, a practice detailed in the EPA Guidance on Testing and Labeling for Drift Reduction Technologies. These polymers increase the elasticity of the fluid, keeping the liquid sheet intact longer as it exits the nozzle. This approach is supported by drift reviews such as Hilz & Vermeer (2013).

Nozzle Performance and Flow Rate: The viscosity of a fertilizer or agrochemical changes the flow rate through hydraulic nozzles. While density affects flow rate via the specific gravity calculation, viscosity alters the discharge coefficient of the nozzle orifice. High-viscosity fluids can cause a pressure drop that reduces the spray angle, leading to streaks or gaps in field coverage.

Laboratory testing must simulate these conditions using flow loops or high-speed imaging. Nozzle performance is often classified using standards like ASABE S572.2. If a formulation is too viscous, it may require specialized air-induction nozzles to achieve proper atomization. Conversely, fluids that are too thin may drift excessively even at low pressures.

Surface Tension Interaction: While viscosity controls the internal resistance to flow, it works in tandem with surface tension to determine wettability. A highly viscous droplet that lands on a leaf may not spread effectively if the surface tension is also high. Formulators must balance viscosity for drift control with surfactants that lower surface tension for coverage.

This interaction is particularly critical for systemic herbicides that need to be absorbed by the plant. If the viscosity prevents the droplet from spreading, the active ingredient remains concentrated in a small area, potentially causing leaf burn. Optimization requires measuring both contact angle and dynamic viscosity.

Optimizing formulation stability and shelf life through viscosity

Long-term stability in agrochemical suspensions is governed by the balance between the fluid's viscosity and the gravitational force acting on suspended particles. According to Stokes' Law, the sedimentation velocity of a particle is inversely proportional to the viscosity of the continuous phase. Therefore, increasing the viscosity of the liquid medium is the most direct method to slow down particle settling and extend shelf life, as described in the Encyclopedia of Colloid and Interface Science.

Stokes’ Law and Sedimentation Velocity: Stokes' Law provides the mathematical foundation for suspension stability. It states that the settling speed depends on the particle radius squared and the density difference between the solid and liquid. By doubling the viscosity of the continuous phase, the settling rate is effectively halved.

However, relying solely on high viscosity can make the product difficult to pour. The industry standard is to create a "structured liquid" that maintains high viscosity at rest (zero-shear) but thins during pouring. This deviation from simple Stokes behavior requires advanced rheological characterization beyond single-point measurements.

Prevention of Hard Packing and Syneresis: When particles do settle, the nature of the sediment is determined by the formulation's rheology. In low-viscosity systems, particles settle quickly and pack tightly, forming a hard cake that cannot be re-dispersed. High-viscosity networks tend to support a "soft flocculation" where particles settle loosely and are easily re-suspended.

Syneresis, or the separation of a clear liquid layer at the top of a container, is another stability failure linked to viscosity. It occurs when the internal network of the fluid contracts. Tuning the concentration of rheology modifiers prevents this separation and ensures the product remains homogeneous from the factory to the field.

Ostwald Ripening and Crystal Growth: Viscosity also plays a role in inhibiting crystal growth, known as Ostwald ripening. In this process, small crystals dissolve and redeposit onto larger crystals, causing them to grow and eventually settle out. A highly viscous continuous phase slows the diffusion of dissolved molecules between crystals, acting as a kinetic barrier to this growth mechanism.

Stabilizers such as lignosulfonates or block copolymers are often used to increase viscosity specifically at the particle interface. Instruments must align with ISO 13320 for particle size analysis to ensure accurate predictions of sedimentation velocity, a key factor often referenced in industry guides such as Croda's Formulating Suspension Concentrates.

Standardized techniques for measuring agrochemical viscosity

Accurate quantification of viscosity requires standardized protocols that account for temperature, shear rate, and equipment geometry. The most common standard for testing non-Newtonian agrochemicals is ASTM D2196, which utilizes rotational viscometers to generate flow curves. These instruments measure the torque required to rotate a spindle immersed in the fluid, providing data that correlates directly to pumping and mixing performance.

Rotational Viscometry (Brookfield Method): The "Brookfield" type viscometer is the industry workhorse for quality control in fertilizer production. By changing spindles and rotational speeds, technicians can measure apparent viscosity at different shear rates. This multi-point testing is essential for detecting shear-thinning behavior that a single-point test would miss.

For fertilizers, it is crucial to use a spindle geometry that is appropriate for the fluid's viscosity range. Using a spindle that is too small for a thick suspension will result in low torque readings and poor accuracy. Conversely, a large spindle in a thin fluid may create turbulence, invalidating the results.

Temperature Control and Consistency: Viscosity is highly temperature-dependent, decreasing as temperature rises. A fluctuation of just 1°C can alter viscosity readings by up to 10% in some concentrated formulations. Therefore, all standard operating procedures (SOPs) for agrochemicals mandate the use of water baths or Peltier plates to maintain a constant test temperature, typically 20°C or 25°C.

Reporting viscosity without stating the temperature renders the data useless. For global distribution, formulations are often tested using methods such as CIPAC MT 192 or ISO 3219 to ensure they remain pumpable in cold climates and stable in hot storage. These thermal profiles are part of the standard FAO specifications for plant protection products.

Sample Preparation and Handling: The history of the sample prior to testing can significantly affect the results, especially for thixotropic fluids. Agitating a sample vigorously before loading it into the viscometer can break down its structure, leading to artificially low readings. Standard protocols specify a resting period after loading to allow the fluid to recover its equilibrium structure.

Additionally, air entrainment during mixing can skew viscosity data. Bubbles act as compressible voids that reduce the torque on the spindle. Vacuum degassing or gentle mixing is recommended to ensure the sample represents the true bulk fluid properties.

Managing the temperature dependence of fertilizer viscosity

The relationship between temperature and viscosity is a critical variable that must be modeled to ensure the field performance of agrochemicals across diverse climates. As the temperature of a liquid fertilizer decreases, its viscosity increases exponentially, potentially exceeding the operational limits of standard application pumps. This "cold flow" problem is particularly acute for high-concentration salt solutions, where low temperatures can also trigger crystallization, creating a slurry that is impossible to spray. Laboratory profiling involves generating viscosity-temperature curves (often using the Arrhenius equation) to define the safe operating window for a product. Conversely, at high temperatures, viscosity may drop low enough to increase drift potential or destabilize an emulsion. Formulators must stress-test products through freeze-thaw cycles to verify that the rheological profile returns to specification once the product returns to ambient temperature.

Conclusion: Mastering viscosity for agrochemical performance

The viscosity of fertilizers and agrochemicals is a master variable that dictates everything from shelf-life stability to spray drift potential. By leveraging advanced rheological characterization, laboratory professionals can predict how complex formulations will behave under the varying shear conditions of storage, pumping, and atomization. Adhering to standards such as ASTM D2196 and conducting rigorous temperature profiling ensures that these critical inputs remain effective and safe for environmental application. Ultimately, a deep understanding of flow dynamics allows the industry to balance the competing demands of drift reduction, biological efficacy, and operational efficiency.

This article was created with the assistance of Generative AI and has undergone editorial review before publishing.